Silica powder has been widely used as an additive to rubbers, paints, etc. Moreover, the silica powder is usually obtained by a wet method in which an alkali silicate is neutralized with a mineral acid or a dry method in which a halogenated silane is decomposed in flame. The silica powder obtained by the former method is also called hydrated silicic acid and is a product usually having a moisture content of 2 to 6%. Since, however, the product is obtained in a wet state, it is ordinarily necessary to reduce the moisture content through the drying procedure as much as possible. While the silica powder obtained by the latter method is called fumed silica and is inherently unhydrated silica.
As the silica powder however has substantial water absorbability, it adsorbs a moisture in an ambient atmosphere and has been supplied as a product of a certain moisture level according to a storage environment and a storage state of the product. It has been so far estimated that the higher the moisture content, the less desirable the product in respect of properties; an attempt has been made to minimize the moisture content. Especially, the fumed silica is inherently unhydrated silica, and an attempt has been made to prevent it from being hydrated as much as possible because of preconception that the lower the moisture content the better or for the reason that when the fumed silica is used by being dispersed in a resin as a filler, wettability between a lipophilic group present in the resin and water present in the silica surface is poor. In fact, the fumed silica is, when contacted with a large amount of water or mist, agglomerated to form grits, degrading the properties. Accordingly, contacting the product with water has been avoided.
In the aforesaid technical background, an attempt to positively control the moisture content of the silica powder by contacting the silica powder with the moisture has not been made nor studied.
Meanwhile, the silica powder finds various uses. It has been found that when the silica powder is used as a filler of a resin, for example, by previously surface-treating it with a surface treating agent such as methyl silane or a silane coupling agent, a given moisture content improves reactivity with the surface treating agent and good results are provided. Development of a technique to control the moisture content of the silica powder which can prevent formation of grits has been therefore demanded.
The present inventors have made assiduous research to develop a technique to positively impart an optional moisture content to a silica powder without impairing the properties, and as a result, surprisingly discovered that an optional moisture content can be imparted to a silica powder without impairing the properties by using a steam and contacting the silica powder with the steam at a temperature lower than a condensation temperature of the steam. This has led to completion of this invention.
That is, it is an object of this invention to impart an optional moisture content to a silica powder without impairing the properties.
Another object of this invention is to provide a technique to control a moisture content of a silica powder by an industrially simple means.
Still another object of this invention is to impart an optional moisture content to a silica powder during transportation of said silica powder by air.
The other objects of this invention will be made clear by the following explanation.